The present invention relates to a color image forming apparatus employing an electro-photographic method.
In general, an image forming method according to an electro-photographic method employed in a color image forming apparatus, such as a copier, printer, etc., includes: forming each of a plurality of primary toner images having different color onto each of a plurality of image bearing members; sequentially transferring each of the unicolor toner images onto an intermediate transfer member to form a full color toner image as primary transferring operation; successively transferring the full color toner image onto a transfer material as a secondary transferring operation; and then, applying a fixing operation to the transfer material to form a final color image.
The electronic potential of the color toner image on the intermediate transfer member varies depending on the attached amount of toner. Accordingly, the secondary transferring condition is normally established at such a value that conforms to the electronic potential of the black toner image in a black solid image. However, it has been a problem that, when a color image having a large amount of attached toner is formed by overlapping a plurality of unicolor toner images, image deficiencies, such as color unevenness and/or a toner scattering, are liable to occur in the final color image as a result of various kinds of transferring errors occurring in the secondary transferring process.
To overcome the abovementioned problem, for instance, there has been proposed such a method that includes: adjusting the electronic potential of the toner layer of the color toner image at a constant potential by applying electronic charge having a polarity same as that of the toner to the color toner image in the secondary transferring pre-precessing to be conducted by means of a scorotron charger including a discharging electrode and a grid electrode before the secondary transferring operation; and then, conducting the secondary transferring operation for the adjusted color toner image (for instance, refer to Tokkaihei 10-274892 (Japanese Non-Examined Patent Publication) and Tokkaihei 11-143255 (Japanese Non-Examined Patent Publication).
However, since the electronic potential of the color toner image on the intermediate transfer member varies depending on not only the attached amount of toner, but also an amount of toner charge, it also varies with changes of the using environment and the using condition. Accordingly, the optimum grid voltage to be applied to the grid electrode varies with the electronic potential of the toner layer of the color toner image to be applied the secondary transferring pre-processing. Therefore, since the grid voltage to be applied to the grid electrode is kept constant in the abovementioned conventional method, it is virtually impossible for the method to always create a good condition for conducting the secondary transferring operation for the color toner image.
[Patent Document 1]                Tokkaihei 10-274892 (Japanese Non-Examined Patent Publication)        
[Patent Document 2]                Tokkaihei 11-143255 (Japanese Non-Examined Patent Publication)        